Sound stimulation of the cochlea leads to mechanoelectric activity in the organ of Corti. Outer hair cells play a central but little understood role in the normal cochlea response to sound. Furthermore, outer hair cell (OHC) activity is controlled in an almost completely unknown way by a complex innervation from the olivocochlear efferent fibers. This proposal seeks to add to our understanding of organ of Corti function by examining its mechanoelectric responses to sound and to efferent nerve electric stimulation. Organ of Corti responses will be measured as gross cochlear potentials, ear canal otoacoustic responses, receptor potentials of hair cells, and most importantly as velocity and displacement responses measured from various locations in the or an of Corti. Each of these measurements provides a different viewpoint on the performance of inner and OHCs acting together in vivo. This proposal applies a dramatic new technology for the study of cellular vibration in the inner ear. Laser feedback interferometry (LFI) is a method with sufficient sensitivity to register the vibration of nearly transparent cellular elements. LFI applied through a microscope allows one to focus a laser beam onto the different cellular structures in the organ of Corti. A detailed study of the traveling wave will provide an important empirical base for theoretical studies of organ of Corti function by providing information on mechanical displacements of its cellular structures. This understanding is necessary in order to determine how mechanical energy stimulates the inner and OHCs. LFI microscopy can provide the cellular displacement measurements needed to determine how OHCs serve as motile elements in this system. The proposal will also study the physiology of the olivocochlear efferent system and determine whether activation of the efferent nerves changes the mechanical properties of OHCs. The physiology of the descending efferent system from the inferior colliculus is included as a way to cause activation of both the medial and the lateral efferent systems in a topographic manned Finally, with this proposal we are beginning the study of the physiology of the supporting cells of the organ of Corti. Until recently supporting cells were thought to be largely passive elements only lending structural support to the system.There is mounting evidence now that Deiters' cells are motile and dynamic structures. We will measure the changes in organ of Corti vibration following types of stimulation that lead to changes in Deiters' cell morphology. Taken together the studies of this proposal will provide an understanding of: how OHCs generate high frequency selectivity and sensitivity in the normal inner ear; the function of the olivocochlear efferent system and the structural dynamics of supporting cells in the organ of Corti.